// 15
package main

import (
	"fmt"
)

func main() {
	var s [21][21]int
	for i := 0; i < 21; i++ {
		for j := 0; j < 21; j++ {
			if i-1 < 0 || j-1 < 0 {
				s[i][j] = 1
			} else {
				s[i][j] = s[i-1][j] + s[i][j-1]
			}
		}
	}
	fmt.Println(s[20][20])
}
